The present invention relates to optical switching apparatuses and more specifically to acousto-optic switches usable in an optical time domain reflectometer.
Optical time domain reflectometers (OTDR) launch optical pulses, generated by an optical transmitter, into a fiber under test to generate an optical return signal from the test fiber. The return signal includes an exponentially decreasing Rayleigh backscatter signal and possibly high amplitude Fresnel reflections from mechanical splices, connectors, or breaks in the fiber. The return optical signal is coupled to an optical receiver having a photodetector, which converts the optical signal into an electrical signal. The analog electrical signal is converted to digital values by an A/D, processed by a controller, and displayed.
Fresnel reflections in the optical return signal generate diffusion currents in the photodetector, which produce, what is known in the art, as detector tail. The detector tail may mask additional events in the fiber and prevents high resolution two-point measurements with the OTDR. To reduce the effects of large Fresnel reflections and increase the performance of the OTDR, traditional optical couplers have been replaced with optical switches. These switches couple the optical transmitter and optical receiver to the fiber under test and, when properly switched, block Fresnel reflections from the optical receiver. Two types of optical switches are currently being used in OTDR's, acousto-optic switches and electro-optic switches. Acousto-optic switches have an acousto-optic modulator (AOM) made of an acousto-optic material, such as TeO.sub.2, LiNbO.sub.2, PbMoO.sub.4, or the like, which generally has a high figure of merit in at least one crystallographic direction. Applying RF energy to the AOM generates acoustic waves in the material, which affects the path of the light passing through the material.
FIG. 1 shows a representative acousto-optic switch as is used in an OTDR. The acousto-optic switch 8 has an acousto-optic modulator (AOM) 10 and three optical ports 12, 14, and 16 with one port 12 acting as a common port. The common port is optically coupled to the fiber under test via an optical waveguide 18 connected to the front panel (not shown) of the OTDR. Second and third ports 14 and 16 are optically coupled via optical waveguides 20 and 22 to the optical transmitter 24, generally a laser diode, and the photodetector 26, generally an avalanche photodiode (APD). Disposed between each optical waveguide 18, 20, and 22 and the AOM 10 are collimating lenses 28, 30, and 32. These lenses collimate the light going into the AOM 10 and focus the light going into waveguides. An optical prism or fold mirrors 34 is included to redirect one of the light paths coming out of the AOM 10. The optical axes of the lenses 28 and 30 are aligned with transmission axes of the AOM 10 and the transmission axes of the waveguides 18, 20 and 22 are aligned with the respective optical axes of the lenses 28, 30 and 32.
The AOM 10 and the prism or fold mirrors 34 are mounted within a milled metal housing. Holes are drilled in the sides of the housing to allow light to pass into and out of the housing. The collimating lenses 28, 30, and 32 and the waveguides 18, 20 and 22 are mounted in collimating lens assemblies and mounted to the outer side surfaces of the switch housing using screws or other such fasteners. A representative collimating lens assembly 40 is shown in FIG. 2. The lens assembly 40 has a elongate sleeve member 42 axially extending from a base member 44. The sleeve member 42 has an axial bore 46 therethrough for receiving an optical waveguide 48. The base member 44 has a central bore region 50 that receives the collimating lens 52. Apertures 54 are formed in the base member 44 that accept threaded screws for mounting the assembly to the outer surface of the switch housing. Generally, collimating lens assemblies 40 are off the shelf purchased parts that are preassembled and optically aligned by the manufacturer. Precise mechanical and optical alignment of the collimating lens assemblies 40, the modulator 10, and the prism or fold mirrors 34 are required in the assembly of the acousto-optic switch 8.
U.S. Pat. No. 4,958,896 describes an optical multi-port element having an acousto-optic modulator where the optical axis of the focusing lenses are aligned to the transmission axis of the acousto-optic modulator and the optical waveguides have their axes spaced from the optical axis of the lenses. The optical waveguides and focusing lenses are mounted in retaining blocks that are adjustably secured to a baseplate by the use of screws or the like. The waveguides and lenses are movable in relation to each other in x-y-z directions. The optical waveguide retaining blocks contain precisely aligned v-grooves that receive the waveguides. The retaining blocks rest on the baseplate and are movable in the x direction relative to the lenses. The retaining blocks for the lenses are movable in both the y and z direction relative to the waveguides.
A major drawback to the above described acousto-optic switches or gates is that the individual optical components are secured in their own mounting receptacles which are then mounted onto a baseplate or housing. Mechanical shock and vibrations can affect the relative alignment of the various optical components causing attenuation or loss of the optical signal. In addition, the various individual mounting receptacles may have different coefficients of thermal expansion causing misalignment of the optical elements due to thermal shock or cycling of the acousto-optic device. Further, the use of mounting receptacles increases the parts count, complexity, and assembly time of the optical switch which increases the cost. Additionally, off axis positioning of the optical waveguides with respect to the optical axis of the focusing lenses reduces the performance of the optical switching apparatus by introducing coma, spherical aberration and the like in the spot size of the focused energy which reduces coupling efficiency.
What is needed is an optical apparatus that overcomes the current limitations of optical switches or gates using individual receptacles for optical components. Ideally, such an apparatus should have all the optical components integrally formed into one housing to lower cost and reduce environmental effects, such as temperature, shake, and shock.